Biomedical subjects

Ajit P Yoganathan

Publications and source records attributed to Ajit P Yoganathan.

At least 19 recordsLinked to original sources

PubMed2006

An ex vivo study of the biological properties of porcine aortic valves in response to circumferential cyclic stretch.

Normal physiological mechanical forces cause constant tissue renewal in aortic valve leaflets (AVL) while altered mechanical forces incite changes in their structural and biological properties. The current study aims at characterizing the remodeling properties of AVL subjected to cyclic circumferential stretch in a sterile ex vivo bioreactor. The leaflets cultured were stretched at a maximum rate of 300%s(-1) corresponding to a 15% strain for 48 h. Collagen, sulfated glycosaminoglycan (sGAG), and elastin contents of the stretched, fresh, and statically incubated leaflets were measured. Cusp morphology and cell phenotype were also examined. AVLs exposed to cyclic stretch showed a significant increase in collagen content (p < 0.05) when compared to fresh and statically incubated AVLs. sGAG content was significantly reduced in the stretched AVLs (p < 0.05) when compared to the fresh leaflets and was comparable between stretched and statically incubated AVLs. There was no statistically significant change in elastin content in all the three groups of AVLs (p > 0.05). Native aortic valve morphology was well preserved in stretched leaflets. Immunohistochemistry and immunoblotting studies showed an increased expression of alpha-smooth muscle actin (alpha-SMA) in stretched leaflets while alpha-SMA expression was reduced in statically incubated AVLs when compared to the fresh leaflets. To conclude, circumferential cyclic stretch altered the extracellular matrix remodeling activity of valvular cells, and consequently the extracellular matrix composition of the AVLs. Most interestingly, the contractile and fibrotic phenotypic expression of valve interstitial cells was enhanced. These results show that circumferential cyclic stretch is a possible mediator for AVL remodeling activity.

Animals↗

PubMed2006

Planar biaxial creep and stress relaxation of the mitral valve anterior leaflet.

A fundamental assumption in mitral valve (MV) therapies is that a repaired or replaced valve should mimic the functionality of the native valve as closely as possible. Thus, improvements in valvular treatments are dependent on the establishment of a complete understanding of the function and mechanical properties of the native normal MV. In a recent study [Grashow et al. ABME 34(2), 2006] we demonstrated that the planar biaxial stress-strain relationship of the MV anterior leaflet (MVAL) exhibited minimal hysteresis and a stress-strain response independent of strain rate, suggesting that MVAL could be modeled as a "quasi-elastic" material. The objective of our current study was to expand these results to provide a more complete picture of the time-dependent mechanical properties of the MVAL. To accomplish this, biaxial stress-relaxation and creep studies were performed on porcine MVAL specimens. Our primary finding was that while the MVAL leaflet exhibited significant stress relaxation, it exhibited negligible creep over the 3-h test. These results furthered our assertion that the MVAL functionally behaves not as a linear or non-linear viscoelastic material, but as an anisotropic quasi-elastic material. These results appear to be unique in the soft tissue literature; suggesting that valvular tissues are unequalled in their ability to withstand significant loading without time-dependent material effects. Moreover, insight into these specialized characteristics can help guide and inform efforts directed toward surgical repair and engineered valvular tissue replacements.

Animals↗

PubMed2006

Fluid dynamic assessment of three polymeric heart valves using particle image velocimetry.

Polymeric heart valves have the potential to reduce thrombogenic complications associated with current mechanical valves and overcome fatigue-related problems experienced by bioprosthetic valves. In this paper we characterize the in vitro velocity and Reynolds Shear Stress (RSS) fields inside and downstream of three different prototype trileaflet polymeric heart valves. The fluid dynamic differences are then correlated with variations in valve design parameters. The three valves differ in leaflet thickness, ranging from 80 to 120 mum, and commisural design, either closed, opened, or semi-opened. The valves were subjected to aortic flow conditions and the velocity measured using three-dimensional stereo Particle Image Velocimetry. The peak forward flow phase in the three valves was characterized by a strong central orifice jet of approximately 2 m/s with a flat profile along the trailing edge of the leaflets. Leakage jets, with principle RSS magnitudes exceeding 4,500 dyn/cm(2), were observed in all valves with larger leaflet thicknesses and also corresponded to larger leakage volumes. Additional leakage jets were observed at the commissural region of valves with the open and the semi-open commissural designs. The results of the present study indicate that commissural design and leaflet thickness influence valve fluid dynamics and thus the thrombogenic potential of trileaflet polymeric valves.

Blood Flow Velocity↗

PubMed2006

Biaixal stress-stretch behavior of the mitral valve anterior leaflet at physiologic strain rates.

Characterization of the mechanical properties of the native mitral valve leaflets at physiological strain rates is a critical step in improving our understanding of MV function and providing experimental data for dynamic constitutive models. We explored, for the first time, the effects of strain rate (from quasi-static to physiologic) on the biaxial mechanical properties of the native mitral valve anterior leaflet (MVAL). A novel high-speed biaxial testing device was developed, capable of achieving in vitro strain rates reported for the MVAL (Sacks et al., Ann. Biomed. Eng. 30(10):1280-1290, 2002). Porcine MVAL specimens were loaded to physiological load levels with cycle periods of 15, 1, 0.5, 0.1, and 0.05 s. The resulting loading stress-strain responses were found to be remarkably independent of strain rate. The hysteresis, defined as the fraction of the membrane strain energy between the loading and unloading curves tension-areal stretch curves, was low (approximately 12%) and did not vary with strain rate. The results of the present work indicated that MVAL tissues exhibit complete strain rate insensitivity at and below physiological strain rates under physiological loading conditions. These novel results suggest that experimental tests utilizing quasi-static strain rates are appropriate for constitutive model development for mitral valve tissues. The mechanisms underlying this quasi-elastic behavior are as yet unknown, but are likely an important functional aspect of native mitral valve tissues and clearly warrant further study.

Animals↗

PubMed2006

In-vivo dynamic deformation of the mitral valve anterior leaflet.

BACKGROUND: Surgical techniques have been developed for mitral valve repair for a wide range of pathologies. However, excessive tissue stress and damage have been identified as etiologic factors limiting long-term durability. Before computational models to optimize valve repair can be realistically developed, in-vivo dynamic mitral valve leaflet strain data are required. However, these data do not presently exist. In the present study, a sheep model and sonomicrometry were used to compute the in-surface Eulerian strain tensor of the anterior leaflet over the cardiac cycle at varying afterloads. METHODS: The anterior leaflet of nine Dorsett sheep (35 kg to 45 kg) was instrumented with nine 1-mm hemispherical piezoelectric transducers in a 15-mm square array. Three-dimensional crystal spatial positions were recorded at 250 Hz over several cardiac cycles, with peak left ventricular pressures varying from 90 mm Hg to 200 mm Hg. The in-surface Eulerian strain tensor was computed from the crystal displacements. RESULTS: The mitral valve anterior leaflet experiences large anisotropic strains and peak strain rates of 400%/s, followed by an absolute cessation of any deformation during systole. Increasing left ventricular pressure also increased the effective leaflet stiffness but not the peak strains. CONCLUSIONS: We report the first data on the dynamic in-vivo strain tensor of a functioning mitral valve anterior leaflet, which indicated large anisotropic strains and very high strain rates. Our observations also suggest that changes in left ventricular pressure and annular geometry result in altered effective leaflet stiffness, and may be an important factor in reducing leaflet stress and as such potentially affect mitral valve repair longevity.

Animals↗

PubMed2006

Effects of annular size, transmitral pressure, and mitral flow rate on the edge-to-edge repair: an in vitro study.

BACKGROUND: Although edge-to-edge repair is an established adjunctive procedure, there is still debate on its long-term durability and efficacy. METHODS: Fifteen porcine mitral valves were studied in a physiologic left heart simulator with a variable size annulus (dilated = 8.22 cm2, normal = 6.86 cm2, contracted = 5.5 cm2). Mitral valves were tested under steady and physiologic pulsatile flow conditions (cardiac outputs: 4 to 6 L/min), at peak transmitral pressures between 100 mm Hg and 140 mm Hg. A miniature force transducer was used to measure the Alfieri stitch force (F(A)). Mitral flow rate (MFR), transmitral pressure, effective orifice area, mitral regurgitation, and F(A) were monitored. RESULTS: The edge-to-edge repair led to a decrease in effective orifice area of 16.55% +/- 8.22%; further reduction in effective orifice area was attained with annular contraction. Mitral regurgitation after the edge-to-edge repair was significantly higher (p <0.05) with annular dilation. In the pulsatile experiments, two peaks in F(A) were observed: one during systole (F(A) = 0.059 +/- 0.024 N) and a second during diastole (F(A) = 0.072 +/- 0.021 N). Multivariate analysis of variance analysis showed that during systole, transmitral pressure and mitral annular area (MAA) had significant effects on F(A) [F(A) = (4.40 x 10(-4)) transmitral pressure (mm Hg) + (5.0 x 10(-3)) MAA (cm2) - 0.05 (R2 = 0.80)], whereas during diastole MFR and MAA had significant effects on F(A) [F(A) = (1.03 x 10(-4)) MFR2 (L/min) - (1.60 x 10(-3)) MAA (cm2) + 0.02 (R2 = 0.90)]. CONCLUSIONS: With annular dilation, mitral regurgitation persisted even after the edge-to-edge repair. The edge-to-edge repair does not cause clinically relevant mitral valve stenosis in a normal size mitral valve. Mitral flow rate and transmitral pressure are the main determinants of F(A) during the cardiac cycle. Increasing annular area increases F(A) during systole but decreases F(A) during diastole. Systolic F(A) may become dominant with increases in MAA or peak transmitral pressure, or both.

Animals↗

PubMed2006

Impaired power output and cardiac index with hypoplastic left heart syndrome: a magnetic resonance imaging study.

BACKGROUND: Unfavorable cardiac mechanics in children with hypoplastic left heart syndrome (HLHS) when compared with other single-ventricle defects may affect long-term morbidity and outcome. Using noninvasive phase contrast magnetic resonance imaging (PC MRI), we examined cardiac mechanics in children with HLHS and compared the results to other single-ventricle defects. METHODS: Eighteen children with HLHS and 18 children with other single-ventricle defects were studied after the Fontan operation. Phase contrast MRI scans were obtained perpendicular to the ascending aorta, and flow was quantified using an in-house segmentation and reconstruction scheme. The total power output was determined using the modified Bernoulli equation along with cardiac output and systemic vascular resistance index. RESULTS: Compared with non-HLHS congenital heart defects, children with HLHS had significantly lower power output (1.40 +/- 0.39 versus 1.78 +/- 0.38 W/m2, p < 0.004) and cardiac index (3.15 +/- 0.97 versus 4.09 +/- 1.23 L x Min(-1) x m(-2), p < 0.009) with a concomitant higher systemic vascular resistance index (28.94 +/- 11.5 versus 22.7 +/- 8.53 WU, p < 0.03) despite generating similar systolic blood pressures (112.9 +/- 22.4 versus 115.2 +/- 23 mm Hg, p > 0.05). CONCLUSIONS: Minimally invasive measurements with PC MRI in children with HLHS showed significantly lower power output and cardiac index when compared with other single-ventricle physiologies. Abnormal aortic flow patterns may contribute to power loss and may have long-term survival and morbidity implications associated with the Fontan procedure. Elevated systemic vascular resistance index despite similar blood pressure opens avenues for therapeutic intervention for afterload reduction.

Adolescent↗

PubMed2006

The material properties of the native porcine mitral valve chordae tendineae: an in vitro investigation.

The material properties of the mitral valve chordae tendineae are important for the understanding of leaflet coaptation configuration and chordal pathology. There is limited information about the mechanical properties of the chordae during physiologic loading. Dual camera stereo photogrammetry was used to measure strains of the chordae in vitro under physiologic loading conditions. Two high-speed, high-resolution cameras captured the movement of graphite markers attached to the central section of the chordae. A uniaxial test simulating the same loading conditions was conducted on the same chordae using the same markers. The maximum strain experienced during the cardiac cycle was 4.29% +/- 3.43%. The loading rate was higher at 75.3% +/- 48.6% strain per second than the unloading rate at -54.8% +/- -56.6% strain per second. The anterior lateral strut chordae had a higher maximum strain (5.7% +/- 3.8%) and loading rate (80.5% +/- 51.9% strain per second) than the posterior medial strut chordae (5.5% +/- 2.3% strain and 68.1% +/- 48.3% strain per second). The posterior medial strut chordae had a higher unloading rate (-68.5% +/- -59.1% strain per second) than the anterior lateral strut chordae (-44.9% +/- -57.2% strain per second). Although the anterior lateral and posterior medial strut chordae have a significantly different diameter and length, they experience a similar strain, strain rate, and tension. In conclusion, a non-destructive technique was developed to measure in vitro chordal strain in the mitral valve. This technique allows the investigation of the behavior of biological tissues under physiologic loading conditions.

Animals↗

PubMed2006

Flow study of an extracardiac connection with persistent left superior vena cava.

BACKGROUND: Numerous studies have sought to optimize the design of total cavopulmonary connections with a single superior vena cava. This study was directed to the 2% to 4.5% of the population with dual superior venae cavae, investigating the flow fields associated with such total cavopulmonary connection anatomies. Additionally, it demonstrates the potential use of computational designs and simulations as surgical planning tools. METHODS: A 3-dimensional model of a total cavopulmonary connection with bilateral superior venae cavae was reconstructed from a patient's magnetic resonance images and investigated experimentally and numerically to assess the power losses and flow structures within the connection. On the basis of these results, a virtual operation was performed in the computer to improve the original connection design. The modified anatomy was studied numerically. RESULTS: Because of a smooth connection with an extracardiac conduit and no major dimension mismatch between the baffle and the connecting vessels, the original anatomy yielded smooth flow fields, low power losses, and few disturbances. However, a large offset between the inferior vena cava and the left superior vena cava resulted in flow stasis and unbalanced hepatic flow distribution. Shifting the inferior vena cava and positioning it between the 2 superior venae cavae resulted in a 7% decrease in power losses and eliminated the associated flow stasis regions in the main pulmonary artery segment. CONCLUSIONS: This study demonstrates the potential use of computer-aided design and numeric simulations for surgical planning. It shows that locating the inferior vena cava between the superior venae cavae may lead to better-balanced lung perfusion. This may require suturing the right and left superior venae cavae closer to each other during the hemi-Fontan or Glenn stage.

Anastomosis, Surgical↗

PubMed2006

Flow and thrombosis at orifices simulating mechanical heart valve leakage regions.

BACKGROUND: While it is established that mechanical heart valves (MHVs) damage blood elements during leakage and forward flow, the role in thrombus formation of platelet activation by high shear flow geometries remains unclear. In this study, continuously recalcified blood was used to measure the effects of blood flow through orifices, which model MHVs, on the generation of procoagulant thrombin and the resulting formation of thrombus. The contribution of platelets to this process was also assessed. METHOD OF APPROACH: 200, 400, 800, and 1200 microm orifices simulated the hinge region of bileaflet MHVs, and 200, 400, and 800 microm wide slits modeled the centerline where the two leaflets meet when the MHV is closed. To assess activation of coagulation during blood recirculation, samples were withdrawn over 0-47 min and the plasmas assayed for thrombin-antithrombin-llI (TAT) levels. Model geometries were also inspected visually. RESULTS: The 200 and 400 microm round orifices induced significant TAT generation and thrombosis over the study interval. In contrast, thrombin generation by the slit orifices, and by the 800 and 1200 microm round orifices, was negligible. In additional experiments with nonrecalcified or platelet-depleted blood, TAT levels were markedly reduced versus the studies with fully anticoagulated whole blood (p < 0.05). CONCLUSIONS: Using the present method, a significant increase in TAT concentration was found for 200 and 400 microm orifices, but not 800 and 1200 microm orifices, indicating that these flow geometries exhibit a critical threshold for activation of coagulation and resulting formation of thrombus. Markedly lower TAT levels were produced in studies with platelet-depleted blood, documenting a key role for platelets in the thrombotic process.

Biomechanical Phenomena↗

PubMed2006

Differential immediate-early gene responses to elevated pressure in porcine aortic valve interstitial cells.

BACKGROUND AND AIM OF THE STUDY: Cardiovascular risk factors are believed to play a role in the pathogenesis of aortic valve disease. In the present study the hypothesis was proposed that elevated pressure would cause a change in the expression of prototypical pro-inflammatory genes. Hence, the expression of MCP-1, osteopontin (OPN), VCAM-1, GM-CSF and PAI-1 was examined using semi-quantitative real-time RT-PCR. METHODS: Porcine aortic valve interstitial cells at passage 1 were exposed to constant pressures of 100, 140, or 170 mmHg or cyclic pressures of 80-120, 120-160, or 150-190 mmHg for 2 h. Static cultures at atmospheric pressure served as controls. Total RNA from pooled experiments was isolated for analysis of gene expression. Single tube primer-mediated RT-PCR was performed directly on the RNA. RESULTS: Cells responded differently to constant and cyclic pressure. The most notable response was the expression of OPN, which was significantly up-regulated under steady conditions but down-regulated under cyclic conditions. The opposite was true in VCAM-1 expression, which was significantly down-regulated at 170 mmHg static pressure, but up-regulated at 140 and 170 mmHg mean cyclic pressure. There was no clear proportional correlation between pressure magnitude and expression of MCP-1, GM-CSF, or PAI-1. However, elevated cyclic pressure caused a proportional increase in VCAM-1 expression and a proportional decrease in OPN expression. CONCLUSION: Elevated cyclic pressure is a potent stimulus for the up-regulation of VCAM-1 expression and the down-regulation of OPN expression. This demonstrates an association between hypertension and aortic valve stenosis and calcification. The regulation of the chemotactic genes MCP-1 and GM-CSF is not correlated to a change in compressive forces.

Animals↗

PubMed2006

Cyclic pressure and shear stress regulate matrix metalloproteinases and cathepsin activity in porcine aortic valves.

BACKGROUND AND AIM OF THE STUDY: It has been shown previously that the exposure of porcine aortic valve leaflets (PAVL) to elevated pressure or steady shear stress increases extracellular matrix (ECM) synthesis. It was hypothesized that exposure of the aortic valves to pressure or shear stress would regulate valvular expression of proteases such as matrix metalloproteinases (MMPs) and cathepsins. METHODS: Fresh PAVLs were exposed to cyclic pressure (170 mmHg at 1.167 Hz) or to a steady shear stress of 25 dyne/cm2 for 48 h, with static culture serving as controls. RESULTS: Shear stress significantly inhibited cathepsin activity and cathepsin L protein expression, but increased MMP-2/9 activity. A cyclic hypertensive pressure condition also slightly decreased MMP-2/9 and cathepsin L activities. CONCLUSION: For the first time, valvular cathepsin and MMP activity were seen to be directly regulated by mechanical forces such as pressure and shear stress. The down-regulation of cathepsin L activity correlates with an up-regulation of ECM synthesis; MMP-2 and -9 activities increased with ECM synthesis under shear stress, showing that these proteinases may serve different roles in valvular remodeling. This observation may have implications for understanding valve biology, as well as for valve tissue engineering.

Animals↗

PubMed2006

Effect of hinge gap width on the microflow structures in 27-mm bileaflet mechanical heart valves.

BACKGROUND AND AIM OF THE STUDY: Most bileaflet mechanical heart valves (BMHVs) incorporate some retrograde flow through their hinge mechanism to prevent flow stasis and inhibit microthrombus formation. This reverse flow is characterized by high velocities and shear stresses, thereby promoting platelet activation and hemolysis inside the hinge region. In the present study, the thromboembolic potential of three 27-mm BMHVs with varying hinge gap widths was assessed via in-vitro characterization of the hinge microflow structures. METHODS: Three 27-mm BMHV prototypes with different hinge gap widths (50, 100, and 200 microm) were provided by St. Jude Medical Inc. The valves were mounted in the mitral position of a left heart flow simulator, and two-dimensional laser Doppler velocimetry was used to measure the hinge velocity fields. RESULTS: All three valve prototypes revealed Reynolds shear stress (RSS) levels above 2000 dynes/cm2, which exceeded the threshold for platelet activation and hemolysis. The hinge flow fields were characterized by leakage jets during systole, and a strong vortical flow during diastole. The leakage jet size and corresponding RSS levels were found to increase with the hinge gap width. All three gap widths had RSS >4000 dynes/cm2 (range: 5640 to 13,315 dynes/cm2). The hinge with the smallest gap width registered the highest jet velocity magnitude (2.08 m/s) during systole. CONCLUSION: The study results showed that the hinge gap width influences washout and RSS levels inside the hinge recess. The 100-microm hinge gap width provided optimum fluid dynamic performance. In contrast, the two valves with large and small hinge gap widths may have higher thromboembolic potential.

Blood Flow Velocity↗

PubMed2005

Total cavopulmonary connection flow with functional left pulmonary artery stenosis: angioplasty and fenestration in vitro.

BACKGROUND: In our multicenter study of the total cavopulmonary connection (TCPC), a cohort of patients with long-segment left pulmonary artery (LPA) stenosis was observed (35%). The clinically recognized detrimental effects of LPA stenosis motivated a computational fluid dynamic simulation study within 3-dimensional patient-specific and idealized TCPC pathways. The goal of this study was to quantify and evaluate the hemodynamic impact of LPA stenosis and to judge interventional strategies aimed at treating it. METHODS AND RESULTS: Simulations were conducted at equal vascular lung resistance, modeling both discrete stenosis (DS) and diffuse long-segment hypoplasia with varying degrees of obstruction (0% to 80%). Models having fenestrations of 2 to 6 mm and atrium pressures of 4 to 14 mm Hg were explored. A patient-specific, extracardiac TCPC with 85% DS was studied in its original configuration and after virtual surgery that dilated the LPA to 0% stenosis in the computer medium. Performance indices improved exponentially (R2>0.99) with decreasing obstruction. Diffuse long-segment hypoplasia was approximately 50% more severe with regard to lung perfusion and cardiac energy loss than DS. Virtual angioplasty performed on the 3-dimensional Fontan anatomy exhibiting an 85% DS stenosis produced a 61% increase in left lung perfusion and a 50% decrease in cardiac energy dissipation. After 4-mm fenestration, TCPC baffle pressure dropped by approximately 10% and left lung perfusion decreased by approximately 8% compared with the 80% DS case. CONCLUSIONS: DS <60% and diffuse long-segment hypoplasia <40% could be considered tolerable because both resulted in only a 12% decrease in left lung perfusion. In contrast to angioplasty, a fenestration (right-to-left shunt) reduced TCPC pressure at the cost of decreased left and right lung perfusion. These results suggest that pre-Fontan computational fluid dynamic simulation may be valuable for determining both the hemodynamic significance of LPA stenosis and the potential benefits of intervention.

Angioplasty↗

PubMed2005

What parameters affect left ventricular diastolic flow propagation velocity? In vitro studies using color M-mode Doppler echocardiography.

BACKGROUND: Insufficient data describe the relationship of hemodynamic parameters to left ventricular (LV) diastolic flow propagation velocity (Vp) measured using color M-mode Doppler echocardiography. METHODS: An in vitro LV model used to simulate LV diastolic inflow with Vp measured under conditions of varying: 1) Stroke volume, 2) heart rate (HR), 3) LV volume, 4) LV compliance, and 5) transmitral flow (TMF) waveforms (Type 1: constant low diastasis flow and Type 2: no diastasis flow). RESULTS: Univariate analysis revealed excellent correlations of Vp with stroke volume (r = 0.98), LV compliance (r = 0.94), and HR with Type 1 TMF (r = 0.97). However, with Type 2 TMF, HR was not associated with Vp. LV volume was not related to Vp under low compliance, but inversely related to Vp under high compliance conditions (r = -0.56). CONCLUSION: These in vitro findings may help elucidate the relationship of hemodynamic parameters to early diastolic LV filling.

Blood Flow Velocity↗

PubMed2005

Mitral valve function and chordal force distribution using a flexible annulus model: an in vitro study.

Since variations in annular motion/shape and papillary muscle displacement have been observed in studies of dilated cardiomyopathy and ischemic mitral regurgitation, the objective of this study was to investigate the effects of annular motion/flexibility and papillary muscle displacement on chordal force and mitral valve function. Six human mitral valves were studied in a left heart simulator using a flexible annular model. Mitral flow, trans-mitral pressure and chordae tendineae tension were monitored online in normal and pathophysiologic papillary muscle positions. The flexible annulus model showed a significant increase in mitral regurgitation volume (p < 0.05) when compared to static annuli models. Furthermore, there was a significant increase of force on the basal chords compared to the force present with the static annuli models. Utilizing the flexible annulus model, papillary muscle displacement significantly increased the force on the anterior strut, posterior intermediate and commissural chords. (1) Papillary muscle displacement increases the tension on the intermediate chords inducing tenting of the leaflets and subsequent regurgitation. (2) The tension on the intermediate and marginal chords is relatively insensitive to annular motion, whereas tension on the basal chords is directly affected by annular motion.

Cadaver↗

PubMed2005

Physics-driven CFD modeling of complex anatomical cardiovascular flows-a TCPC case study.

Recent developments in medical image acquisition combined with the latest advancements in numerical methods for solving the Navier-Stokes equations have created unprecedented opportunities for developing simple and reliable computational fluid dynamics (CFD) tools for meeting patient-specific surgical planning objectives. However, for CFD to reach its full potential and gain the trust and confidence of medical practitioners, physics-driven numerical modeling is required. This study reports on the experience gained from an ongoing integrated CFD modeling effort aimed at developing an advanced numerical simulation tool capable of accurately predicting flow characteristics in an anatomically correct total cavopulmonary connection (TCPC). An anatomical intra-atrial TCPC model is reconstructed from a stack of magnetic resonance (MR) images acquired in vivo. An exact replica of the computational geometry was built using transparent rapid prototyping. Following the same approach as in earlier studies on idealized models, flow structures, pressure drops, and energy losses were assessed both numerically and experimentally, then compared. Numerical studies were performed with both a first-order accurate commercial software and a recently developed, second-order accurate, in-house flow solver. The commercial CFD model could, with reasonable accuracy, capture global flow quantities of interest such as control volume power losses and pressure drops and time-averaged flow patterns. However, for steady inflow conditions, both flow visualization experiments and particle image velocimetry (PIV) measurements revealed unsteady, complex, and highly 3D flow structures, which could not be captured by this numerical model with the available computational resources and additional modeling efforts that are described. Preliminary time-accurate computations with the in-house flow solver were shown to capture for the first time these complex flow features and yielded solutions in good agreement with the experimental observations. Flow fields obtained were similar for the studied total cardiac output range (1-3 1/min); however hydrodynamic power loss increased dramatically with increasing cardiac output, suggesting significant energy demand at exercise conditions. The simulation of cardiovascular flows poses a formidable challenge to even the most advanced CFD tools currently available. A successful prediction requires a two-pronged, physics-based approach, which integrates high-resolution CFD tools and high-resolution laboratory measurements.

Blood Flow Velocity↗

PubMed2005

A comparison of flow field structures of two tri-leaflet polymeric heart valves.

Polymeric heart valves have the potential to reduce thrombogenic complications associated with current mechanical valves and overcome fatigue-related problems experienced by bioprosthetic valves. In this in vitro study, the velocity fields inside and downstream of two different prototype tri-lealfet polymeric heart valves were studied. Experiments were conducted on two 23 mm prototype polymeric valves, provided by AorTech Europe, having open or closed commissure designs and leaflet thickness of 120 and 80 microm, respectively. A two-dimensional LDV system was used to measure the velocity fields in the vicinity of the two valves under simulated physiological conditions. Both commissural design and leaflet thickness were found to affect the flow characteristics. In particular, very high levels of Reynolds shear stress of 13,000 dynes/cm2 were found in the leakage flow of the open commisure design. Maximum leakage velocities in the open and closed designs were 3.6 m/s and 0.5 m/s respectively; the peak forward flow velocities were 2.0 m/s and 2.6 m/s, respectively. In both valve designs, shear stress levels exceeding 4,000 dyne/cm2 were observed at the trailing edge of the leaflets and in the leakage and central orifice jets during peak systole. Additionally, regions of low velocity flow conducive to thrombus formation were observed in diastole. The flow structures measured in these experiments are consistent with the location of thrombus formation observed in preliminary animal experiments.

Animals↗